1. Field of the Invention
The present invention relates to an industrial scale treatment of soluble thorium compounds to convert same into a thorium phosphate that is both insoluble and non-leachable, with a view to controlling polluting problems related to discharges and/or storage of thorium products and other radioactive materials.
2. Description of the Prior Art
It is known to this art that certain industries, in particular those which are based on the hydro-metallurgical treatments of ores, can produce, when the treated ores contain, inter alia, thorium (monazite, uranothioranite, for example), various natural thorium salts, such as, generally, the chlorides or nitrates which are highly soluble in water.
Taking account, in particular, of their intrinsic radioactivity, these thorium salts have, unfortunately, to date been used only sparingly. Consequently, and while awaiting future potential outlets for thorium, the current production surpluses, taken as a whole, amount to simple byproducts which today have no added value and which are stored with the greatest care and at great expense, to prevent any harm to man and/or the environment. In this respect, it should be noted that, in terms of radiotoxicity, thorium-232which is the parent radioisotope of the thorium family, is currently subjected to health standards, especially at the level of its presence in water, which are extremely strict and severe.
From this fact, the present inventors reasoned that the above problems, especially storage, could indeed be limited, and even removed, if it were possible to convert all of the thorium values, initially present in the form of one of its soluble salts, for example, in effluents, residues, wastes and similar materials, into a final compound which is as insoluble and as non-leachable as possible, namely, into a form which is completely inert, in particular to the environment. Not only the effluent generated by the insolubilization operation would thus be freed of the undesirable thorium values and could then optionally be enhanced in value, but, additionally, the recovery of the thorium would then be carried out in a directly storable form and in a reliable and economic manner.
Given that the complete insolubilization of the thorium constituted a necessary precondition to attain storage which is both economic and which has, on a long term basis, no noxious or harmful impact on the environment, the present inventors then considered insoluble compounds of the thorium phosphate type. Indeed, these compounds have been shown to be the thorium salts which have the weakest solubilities. Consequently, to succeed in perfectly controlling, on an industrial scale the synthesis of thorium phosphates from soluble thorium compounds it would be required to simultaneously provide (i) effluents which have been freed of this species, and thus potentially added in value, (ii) for the storage of thorium in its most insoluble, and thereby most inert, form and, finally, optionally, (iii) for the use of the phosphates thus prepared as a coating and/or protective material for storing other radioactive wastes.
It will be appreciated that processes for the preparation of compounds of the thorium phosphate type are known to this art. Thus, it is known that such compounds can be obtained by direct reaction between, on the one hand, a solution of a soluble thorium salt (nitrate, chloride) and, on the other, an alkali metal phosphate or phosphoric acid (see, especially, Paul Pascal, Nouveau Traite de Chimie Minerale [New Treatise on Organic Chemistry], Volume IX, page 1132 et seq (1963)). However, in all cases, gels are produced which are extremely difficult, indeed impossible, to filter. Consequently, such synthetic routes cannot be used on a truly industrial scale.